A polarizing plate has been usually used in a structure, in which a protective film is stacked on both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine. In this case, a triacetyl cellulose (TAC)-based film has been frequently used as the protective film because the film is excellent in optical transparency or moisture permeability.
Meanwhile, as a liquid crystal display device has recently been developed into a mobile device such as a laptop personal computer, a cellular phone and a car navigation system, it is required that a polarizing plate which constitutes a liquid crystal display device is thin and light weight. However, in a polarizing plate in which a TAC film and the like are laminated as the protective film as described above, it is difficult to maintain a thickness of the protective film at 20 μm or less from the viewpoint of handling property or durability performance during the operation, so that there is a limitation for the liquid crystal device to be thin and light weight.
In order to solve the aforementioned problems, a technology has been suggested, in which a protective film is provided on only one surface of a polarizer, and a transparent thin film layer is formed by coating an active energy ray curable composition on the other surface thereof. Meanwhile, the active energy ray curable composition which has been suggested may be classified into a radical curable composition and a cationic curable composition according to the curing method. In this case, when a transparent thin film layer is formed using a cationic curable composition, there is an advantage in that excellent adhesion with the polarizer is achieved, but there are many disadvantages in the manufacturing process due to a slow curing rate and a low degree of curing.
In order to solve such problems of the cationic curable composition, a technology has been suggested, in which a transparent thin film layer is formed by using a radical curable composition which includes an acrylic or acrylamide-based compound as a main component. However, in the case of a radical curable composition including an acrylic or acrylamide-based compound as a main component, there are problems in that the curing rate is fast compared to a cationic curable composition, but the curing rate becomes slow under a high humidity atmosphere, and water resistance is not good such that a transparent protective layer formed by using the same does not have sufficient adhesion with a polarizer, and iodide ions in the polarizer are affected. Further, in the case of a radical curable composition including the acrylic or acrylamide-based compound as a main component, there is a problem in that the heat resistance reliability of a polarizing plate including the same is not good because a protective layer has a low glass transition temperature after curing.
Therefore, there is a need for a new polarizing plate, which is excellent in adhesion between a polarizer and a protective layer, and is excellent in water resistance and heat resistance while having a thin protective layer which may be formed by curing radicals.